"Bigger. Badder. Boa," declares the banner of the swanky Smithsonian titanoboa exhibit in Grand Central Station, New York City. The replica of the ancient, and, thankfully for humans, extinct, snake measures 48 feet long. Its diameter is that of a manhole. The sinewy hulk currently lies on the cold marble of Grand Central, jaw-loosened, draconic head arcing back as it swallows a spiny crocodile, its hexagonal scales gleaming with venue lighting and overhead chandeliers.

The replica of the 2,500-pound snake was unveiled today at the commuter hub to stimulate public interest in science, and promote the Smithsonian's more extensive titanoboa exhibit in D.C., which opens next week.

Comparison to modern snakes is even starker. The world's longest snake, the reticulate python, stretches a little more than half the length of the titanoboa. The green anaconda, the world's heaviest snake, is about a tenth of its weight.

Primeval hugeness isn't exclusive to snakes, either. Prehistoric animals seemed to have been much, much larger than they are today. The plant-eating Argentinosaurus is thought to have measured more than 100 feet long and weighed over 100 tons. The Quetzalcoatlus had a 45-foot wingspan. The water-bound Icththysaur was roughly 50 feet long, weighing 30 tons, a rough hybrid of a dolphin and a blue whale. The Ground Sloth was the size of today's elephant.

What's with this trend? Do we know why prehistory was so redolent with animals that dwarf those we see today?

Well, first, not all prehistoric animals were giant. Woolly mammoths aren't thought to have been significantly larger than today's elephant. The horse's earliest known ancestor is thought to have been the size of a house pet. And let's not forget that the largest animal ever, the krill-siphoning, 200-ton blue whale, is still with us today, although it is endangered.

Given those exceptions, though, prehistoric animals were markedly big, and scientists continue to ponder why.

Large animals tend to be better predators and worse prey. After all, who is going to hunt and eat a titanoboa? Even non-predatory animals used their size to compete for resources, like plants. Big animals tend to live longer and reproduce less often, which causes them to evolve more slowly than other species. The perks of their size, however, made them particularly vulnerable to wide-scale extinction events. After such a cataclysm, it can take millions of years for huge animals to reappear.

Body size may have also been a response to the changing environment. There's modern evidence that suggests animals are shrinking due to the warming planet. Inversely, when the ice ages occurred, warm-blooded animals increased in size to retain heat. Cold-blooded ones favored large bodies in warmer climates to better insulate them from overheating.

Another theory suggests that size, particularly in dinosaurs, was fueled by rampant vegetation. Some 65 million years ago, in the Cretaceous period, atmospheric carbon dioxide was much more prevalent than it is today (note that this is not contradictory to anthropogenic global warming, which is operating on a much shorter timescale). Increasing carbon dioxide levels tend to drive up temperature, and so much of the Cretaceous world is thought to have been carpeted with thick vegetation. With this as their food base, dinosaurs weren't forced to shrink into forms that required less food, less energy, as there was plenty for them to eat.

What about living things that aren't animals? The various scientific quests to discover the world's largest organism may not have to delve into prehistoric time. Even the blue whale doesn't compare to the 275-foot General Sherman Sequoia, with a trunk that weighs over 2,000 tons. And then there is the grove of genetically identical aspen trees in Fishlake National Forest – considered by some scientists to count as a single organism – which span 106 acres, and is estimated to weigh over 6,000 tons.